This invention concerns itself with the seals on air cushion vehicles and more particularly with the seals in the front and the rear of air cushion vehicles with rigid sidewalls. These air cushion vehicles are also known as "captured air bubble" (CAB) vehicles, and the rigid sidewalls are also referred to as "sidehulls". The function of the front and rear seals is of course to contain, together with the sidewalls, the air cushion under the vehicle.
The air cushion vehicle with rigid sidewalls is mostly used for over-water service with little or no amphibious capability. The reason for this is that the rigid sidewalls cannot conform to an uneven land-contour but are satisfactory for over-water travel, this because their slender shape allows them to submerge in water with only relatively little hydrodynamic drag when encountering waves.
The invention will be described as designed for an over-water sidewall type air cushion vehicle, but it must be clearly understood than its usage is not limited to this application and that it can be used also on other types of air cushion vehicles and devices as well as vehicles and devices that do not use air but other gaseous or fluid media to support at least part of their weight.
The most simple way to seal off an air cushion in the front and the rear is to use stiff hinged flaps as shown in FIG. 1 and FIG. 3 of U.S. Pat. No. 3,191,705 (Jones and Hardy). However, the cushion pressure will tend to push these flaps into an open position letting the cushion air escape. When the vehicle is moving, the hydrodynamic planing forces on the front seal will counteract this opening force, but to prevent the seal from being pushed open when the vehicle is stopped or moving very slowly, physical "stops" have to be provided limiting its forward position. U.S. Pat. No. 3,191,705 shows such stops in its FIG. 3, item 6, consisting of two blocks attached to the sidewalls stopping the seal at its ends. On the rear seal any hydrodynamic planing forces will act with the cushion pressure and therefore a counteracting force has to be created. In U.S. Pat. No. 3,191,705, FIG. 3, this is done by suspending a membrane, item 10, between the end of the seal and the ship structure, and blowing pressurised air in the cavity between the membrane, the seal flap and the ship structure. If the pressure in this cavity is kept above cushion pressure, it will keep the seal down.
A major problem with these stiff flap seals is that they do not conform to changes in waterheight across the ship (perpendicular to the ships lengthwise centerline). This will cause first of all substantial air leakage under the part of the seal where the waterlevel is lower. Second, when the vehicle moves over a wave with the ship direction not perpendicular to the wave crest, the seal flap will be pushed up at one corner and due to its stiffness, the force required to accelerate its entire mass will be exerted on this corner. These acceleration forces are very large when traveling at high speed over waves, and will subject the stiff flap structure to high torsional loads and stresses. Making the seal structure stronger will tend to be a self-defeating solution, as it will increase the seal's weight, which will increase again the acceleration forces developed. In practice these acceleration forces will destroy any stiff seal flap structure, unless so heavy that it cannot follow waves sufficiently and will create unacceptable high drag.
A solution to the above problem is to let the seal consist of a substantial number of approximately equidistant rods (or "stays") hinged from the ship structure like the flap, but connected by flexible membranes, as shown in FIG. 2 of U.S. Pat. No. 3,532,180 (Ford and Wilson). The "stays" consist here of spring steel strips 33 sewn or bonded within a rubberised fabric or similar material 35. As the seal is now compliant across the ship, it is however not possible anymore to have simple fixed stops at the ends of the seal as shown in the previously mentioned U.S. Pat. No. 3,191,705. Therefore U.S. Pat. No. 3,532,180 uses so-called "down-stop lines" (items 51 in its FIGS. 1, 2 and 3) that tie each individual stay to the ship structure above.
This seal design, though an improvement over the previously described stiff flap, has not been satisfactory either. The main problem is that the down-stop lines get slack when the waves push the seal up and then tighten up again after the wave has passed. The downward movement of the seal after passing a wave has to be fast (at high vehicle speed) to keep air leakage at a minimum, and when the down-stop lines get tight they induce large shockloads on the rods and the attachments of the membranes to the rods, causing both breakage and membrane tearing. In addition, this type of seal--essentially one large membrane with stiffening rods--is subject to flutter-type vibrations. That is, after passing over a wave and loosing contact with the water it will flap around like a flag in the wind, causing neighbouring rods to move in opposing directions till the membrane gets tight with a shock, causing tear failures of the membrane. This structural integrity deficiency was very well demonstrated on the U.S. Navy's 100A (100 ton) sidewall type air cushion test craft that was initially outfitted with a bow seal of this design. (Note: The "flutter" problem has been reduced to a certain extent in U.S. Pat. No. 3,532,180 by making the upper part of the seal, item 27 in their FIG. 3, a stiff plate, however by doing so the problems of the stiff plate as discussed before are reintroduced. The 100A bow seal did not have the upper part of the seal a stiff flap, but had stiffening rods all the way from the bottom to the top of the seal hinged to the ship structure and connected by a membrane.)
The object of this invention is to provide seals of the just discussed kind, that is with a substantial number of more or less equidistant hinged rods (or "stays") connected by flexible membranes, that have means to keep the seal in a nominal position, these means being such that:
(1) They do not induce the previously mentioned shock loads on rods and membranes. PA1 (2) They allow the seals to follow waves upwards without excessive hydrodynamic drag and downwards without excessive air leakage from the cushion. PA1 (3) They allow the pilot of the ship to change the nominal position of the seals continuously between a down position with the lower hemline of the seals approximately flush with the bottom of the side walls and a fully up position with the seals approximately horizontal, without adversely affecting the wave-following capabilities mentioned in point (2). PA1 (4) They allow the pilot of the ship to change the reaction forces on the ship from the seals depending on sea state, speed and heading of the ship, to minimize pitch, roll and heave motions of the ship.